Pain and Temperature Pathways
Receptors, Neurons, and Lateral Spinothalamic Tract
Receptors
The sensory pathways that deliver pain and temperature information to the brain are each composed of a receptor and three successive neurons.
A receptor is the initial portion of the first neuron in a pathway that is sensitive to changes in the environment (i.e., stimuli). Pain receptors (nociceptors) and temperature receptors (thermoreceptors) are unencapsulated and branched, like the fine roots of a plant.
Because nociceptors and thermoreceptors contain no specialized structures, they are often referred to as free nerve endings. They are especially abundant in the skin but occur elsewhere in the body. Internal nociceptors are found in a variety of organs, such as muscles, joints, the bladder, and the digestive tract.
First-Order and Second-Order Neurons
The first neuron in a pain or temperature pathway is called a sensory neuron (or first-order neuron). An axon of a sensory neuron passes through the dorsal root ganglion and extends to the dorsal horn of the spinal cord, where it synapses with a second-order neuron (or interneuron). The term interneuron applies to any neuron within the central nervous system.
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Lateral Spinothalamic Tract
The axon from the second-order neuron decussates (crosses) within the spinal cord and ascends in the lateral spinothalamic tract, along with other second-order neurons.
The spinothalamic tract axons pass through the medulla oblongata region of the brain before terminating at the thalamus’s ventral posterolateral (VPL) nucleus.
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Third-Order Neurons
In the thalamus, the second-order neuron synapses with a third-order neuron (interneuron). The axon of the third-order neuron then carries signals to the primary somatosensory region of the cerebral cortex, where the input is imaged and interpreted.
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References and Attributions
OpenStax, Anatomy and Physiology – “Central Processing.” Access for free at – https://openstax.org/books/anatomy-and-physiology/pages/1-introduction
Wikipedia – “Spinothalamic Tract.”